MASS TRANSFER AND MASS TRANSFER OPERATIONS

Mass transfer

• Mass transfer means the transfer of substance through another on a molecule scale.
• The net movement of mass from one location usually by a stream, phase, fraction, or component, to another.
• This mass transfer occurs due to the concentration difference or gradient.

Hence The process of transfer of mass as a result of the concentration difference of a component in a mixture or two-phase which are in contact is called mass transfer

• Example:- evaporation of water from a pool of water into a stream of air flowing over the water surface

Mass transfer operations

The operation carried out for separating the component of the mixture, involving the transfer of (mass) material from one homogeneous phase to another, due to the difference in vapor pressure, solubility or diffusivity and in which driving force for transfer is a concentration difference is called mass transfer operations

• In the mass transfer operations, neither equilibrium phase consists of only one component. 
• Hence when two-phase is initially contacted, they will not be of equilibrium compositions. 
• The system then attempts to reach equilibrium by a relatively slow diffusive movement of the constituents, which transfer in part between the phases in the process. 
• Separations are therefore never complete, although they can be brought as near completion as desired. 
• Mass transfer occurs in many processes such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. 

In mass transfer operations, the Mass transfer may occur

• In one direction (gas absorption)
• In both direction (distillation)
• With simultaneous heat transfer( drying and crystallization)
• With simultaneous chemical reaction (Absorption of CO2 in an aqueous solution of KOH)
• With the exchange of one or more components
• Isothermally and non-isothermally

MODES OF HEAT TRNSFER

Modes of heat transfer

Heat transfer may be defined as the transmission of the energy from one region to another region as a result of the temperature gradient.

Heat transfer takes place in three modes.

1. Conduction

Conduction is the transfer of heat from one part of substance to another part of same substance or from one substance to another which is in physical contact with it. 

• While transferring heat there is no appreciable displacement of molecules forming that substance is occur.
• In solid, the flow of heat is result of vibrational energy.
• In liquid, the flow of heat is result of kinetic energy.

2. Convection

Convection is the transfer of heat within a fluid which is arises due to mixing of one portion of the fluid with another. 

• In a fluid, the flow of heat results in the microscopic motion of a fluid.
• There are two types of Convection.

1. Natural Convection:- Heat transfer is caused by differences in density arising from a temperature gradient
2. Forced Convection:- Heat transfer is caused by use of external means of agency

3. Radiation

Radiation is the transfer of heat through space or matter by means other than conduction or convection.

• Transfer of heat is occurred without any material medium or through a perfect vacuum.
• Transfer of heat is in the form of electromagnetic waves.
• When this wave falls on another body, a part of the energy is reflected, transmitted through the body and remaining absorbed by the body.

Figure of Conduction Convection and Radiation